PSI - Issue 2_B

V. Crupi et al. / Procedia Structural Integrity 2 (2016) 1221–1228 Author name / Structural Integrity Procedia 00 (2016) 000–000

1228

8

5. Conclusion The results of structural investigation allow us to make a following conclusion. The increase of the number of cycles under ultrasonic fatigue test leads to the sample dilatation. The dilatation can be caused by initiation of new dislocations, micro voids and cracks. The hydrostatic weighing shows that the maximum of dilatation is observed in the middle part of the iron samples. The high initial concentration of these defects in metals and their initiation during initial stage of deformation process allows us to propose the importance of the consideration of their evolution under cyclic loading. The small size and high concentration of submicrocracks in metals, the existing of their size and orientation distributions allows us to develop a statistical description of microcrak evolution in metals under cyclic loading and introduce a new thermodynamical variable – defect induced strain. The new variable gives a natural description of thermodynamics of metals with microcracks and allows one to describe the interaction of plasticity and failure processes. The combination of statistical description of microcrack ensemble with stochastic consideration of defect initiation process allows us to describe the effect of initial nucleus concentration of the deformation process. This model coupled with a description of nonlocal effect in the defect ensemble gives us a key parameter for the description of defect kinetics in the bulk and near specimen surface under cyclic loading. The model illustrates the basic physical mechanisms of damage to fracture transition under fatigue loading in the bulk and near specimen surface and calculate the corresponding temperature evolution. It was shown that the stress amplitude can influence on the location of macro fatigue crack initiation. At small stress amplitude the defect induced strain reaches an equilibrium value near specimen surface due to the defect diffusion and annihilation processes. It can be considered as an infinite fatigue life but in this case there is possibility of blow up regime of defect kinetics in the volume of the specimen. It leads to the shift of the location of the crack initiation from the surface to the volume of the specimen. Acknowledgements The work was funded by RFBR according to the research project No.14-01-00122, No.14-01-96005. References Murakami Y., Nomoto T., Ueda T., 1999. Factors influencing mechanism of superlong fatigue failure in steel. Fatigue & Fracture of Engineering Materials & Structures 22, 581-590. Bathias C., Paris P., 2004. Gigacycle Fatigue in Mechanical Practice, Taylor & Francis, pp. 328 Zhu X., Shyam A., Jones J.W., Mayer H., Lasecki J.V., Allison J.E., 2006. Effect of microstructure and temperature on fatigue behavior of E319-T7 cast aluminum alloy in very long life cycles. International Journal of Fatigue 28, 1566-1571. Liu X., Sun Ch., Hong Y., 2015. Effects of stress ratio on high-cycle and very-high-cycle fatigue behavior of a Ti–6Al–4V alloy. Materials Science and Engineering: A. 622, 228-235. Mayer H., Schuller R., Karr U., Irrasch D., Fitzka M., Hahn M., Bacher-Höchst M., 2015. Cyclic torsion very high cycle fatigue of VDSiCr spring steel at different load ratios. International Journal of Fatigue 70, 322-327. Nikitin A., Bathias C., Palin-Luc T., Shanyavskiy A., 2016. Crack path in aeronautical titanium alloy under ultrasonic torsion loading. Frattura ed Integrita Strutturale 10, 213-222. Ranc N., Favier V., Munier B., Vales F., Thoquenne G., Lefebvre F., 2015. Thermal Response of C45 Steel in High and Very High Cycle Fatigue. Procedia Engineering 133, 265-271 Plekhov O., Naimark O., Semenova I., Polyakov A., Valiev R., 2015. Experimental study of thermodynamic and fatigue properties of submicrocrystalline titanium under high cyclic and gigacyclic fatigue regime”, Proceedings of the Institution of Mechanical Engineers Part C - Journal of Mechanical Engineering Science 229, 1271-1279. Crupi V., Epasto G., Guglielmino E., Risitano G., 2015. Thermographic method for very high cycle fatigue design in transportation engineering, Proceedings of the Institution of Mechanical Engineers Part C - Journal of Mechanical Engineering Science 229, 1260-1270. Crupi V., Epasto G., Guglielmino E., Risitano G., 2015. Analysis of temperature and fracture surface of AISI4140 steel in very high cycle fatigue regime. Theoretical and Applied Fracture Mechanics 80, 22-30 Naimark O., 2003. Defect Induced Transitions as Mechanisms of Plasticity and Failure in Multifield Continua In “Advances in Multifield Theories of Continua with Substructure”, Birkhauser Boston, Inc., pp. 75-87. Doudard C. Calloch S., Cugy P., Galtier A., Hild F., 2005. A probabilistic two-scale model for high-cycle fatigue life predictions. Fatigue & Fracture of Engineering Materials & Structures 28, 279–288.